This invention relates in general to detecting errors in orientation of media to be printed and in particular to detecting alignment errors in lenticular material prior to printing.
When a sheet fed printer is loaded with media having different surface characteristics on both sides, such as lenticular media, there are many opportunities for error in terms of flipping the media the wrong way up before it is printed. Other errors include curling, buckling, skewed, misaligned, and missized sheets. The printing of quality images requires correct orientation of the media in the printer.
The slitting, cutting, packaging, and printer loading operations may be manual operations that take place in the dark and it is not trivial to verify that the media is loaded the correct way up in the printer. As the printing operation occurs unattended in the dark, particularly for photosensitive material, an error might not be noticed until the media is developed. If an entire stack of media is printed upside down, there is potential for significant economic loss.
U.S. Pat. No. 5,414,491, Vacuum Holder for Sheet Materials, shows a method of determining different sheet sizes based on changes in vacuum flow. This patent, however, does not provide a method and apparatus for distinguishing misalignments of sheets or other problems described above.
It is therefore desirable to provide a means for detecting errors in loading lenticular material on printers.
Briefly, according to one aspect of the present invention, a method for detecting errors in loading a lenticular material on a printer comprises loading the lenticular material on a vacuum platen and drawing a vacuum on the vacuum platen. An airflow is measured on the vacuum platen and compared to a predetermined value.
The lenticular media, which is used as an illustrative example, has a photographic emulsion on one side of a support and lenticular lenses on the other. When placed on a vacuum platen, the amount of flow in the vacuum lines can be used as an indication of errors in loading. A nominal xe2x80x9ccorrectxe2x80x9d value is stored to be used as a reference from which to make decisions. This nominal value is stored during initial system setup, when media is placed correctly on the vacuum platen and the source of the system vacuum is in a known state. After storing this nominal value, two possible classes of errors can be differentially distinguished; an error in placement or seating (not all of the vacuum holes in the platen covered with media), or an error in media loading (the media is loaded with the emulsion down rather than the lenticals down). In other words, if the media is loaded with the emulsion on the platen, it""s smooth surface mates better with the vacuum platen and does not allow any appreciable flow in the vacuum lines. This results in a value for the flow that is less than the nominal value. If, however, the media is loaded correctly with the lenticules facing the platen, a small amount of flow in the vacuum lines is present due to leakage along the clefts of the lenticals, which can be measured and compared to the nominal value to verify that the media is placed on the platen properly. If the media has either not been seated properly on the platen, or is skewed, some of the vacuum holes will be left open resulting in a greater than nominal flow measurement which can be detected.
In addition, this kind of sensing could identify other major errors such as the wrong media size or type. The following table provides a summary of the sensing states:
No (or very little flow): Media loaded upside down (error)
Moderate flow: Media loaded correctly
High flow: Media not seated on platen properly, media skewed, or wrong media size (error)
Setting the thresholds between these states in the software controlling the printer depends on the vacuum system used, the lenticular dimensions, the platen hole pattern, the sensor used to detect flow, and the electronics used to condition the raw sensor signal. A low pass filter is used in the implementation to reduce the effects of higher frequency signal noise that is prevalent in such signals, due to sensor characteristics as well as pneumatic system noise. A low pass filter can be implemented in hardware or firmware, such as a moving average signal processing technique.
Measuring flow is preferred over a simple vacuum pressure measurement due to the S/N ratio achievable. If pressure in the lines were measured, only a small change in signal would be noted, because the vacuum source can maintain the same nominal pressure in the lines regardless of flow. However, the amount of leakage in the platen, and therefore flow in the vacuum lines will produce a significant signal change from the appropriate flow sensor and so, can be used as a reliable sensing method.
Various commercially available instruments can be used to measure flow. The Pitot tube is a common method in which the flow is converted to a static stagnation pressure that can be easily measured. This is the method that has been demonstrated. In this configuration, there is still a relatively small pressure developed by the flow, on the order of 5 inches of water, (125 mm Hg).
As alternatives to the preferred Pitot tube embodiment, a traditional anemometer could also be used to measure the flow rate. Alternatively xe2x80x9chot wirexe2x80x9d anemometers are available.